This invention relates to the method and means for stimulating wells by injecting fluid in response to the detonation of a high explosive.
In an application Ser. No. 250,184 by Charles S. Godfrey, filed May 4, 1972, now abandoned, which is assigned to a common assignee, there is described a method and means for fracturing the rock formation in a well at a predetermined depth in the well. The reason for causing these fractures is to stimulate these wells to increase the flow rate of gas or oil. If these fractures can be formed in a manner to radiate in all directions from the wellbore, then there is a good likelihood for oil or gas to flow into the fractures and to the wellbore. The aforementioned application describes a technique for fracturing the rock formation, which includes generating a detonation which applies a shock wave to the fracturing fluid, which has a rise time which is less than the time required for sound to traverse one-half of the periphery of the wellbore, and the amplitude of the shockwave should be less than the amount which will crush the rock formation.
The reason for the requirement that the shock wave rise time be so rapid is because this is what sets up a rapid rise time stress wave in the rock formation, which causes multiple radial fractures. With a slow rise time stress wave, such as is produced by a mechanical pump or deflagration of a propellent type of explosive, a single fracture through the borehole would relieve the stress to the extent that no more fractures can occur. With a fast rise stress wave, full stress is applied to the rock strata around the hole before a single fracture can occur to relieve the stress and thus multiple radial fractures can be caused to occur rather than one.
After a fracture has been created, it is desirable that that fracture extend as deeply as possible in order to reach the producing region surrounding a wellbore. In order to extend a fracture there must be a source of energy applying pressure to the fluid driven by the initial detonation into the fracture caused thereby. Accordingly, if it were possible to provide a detonation having the indicated rise time and amplitude to cause a plurality of radial fractures in the pay zone, and to thereafter maintain the pressure required to extend the fractures over an interval which is long when compared to the interval, normally in microseconds, over which the detonation pressure exists, radial fractures can be extended considerably more with a greater likelihood for a pay off.